Welcome to Log4j 2!

Introduction

Almost every large application includes its own logging or tracing
API. In conformance with this rule, the E.U. SEMPER project decided to write its
own tracing API. This was in early 1996. After countless enhancements,
several incarnations and much work that API has evolved to become
log4j, a popular logging package for Java. The package is distributed
under the Apache Software License, a
fully-fledged open source license certified by the open source initiative. The
latest log4j version, including full-source code, class files and
documentation can be found at http://logging.apache.org/log4j/2.x/index.html.

Inserting log statements into code is a low-tech method for
debugging it. It may also be the only way because debuggers are not
always available or applicable. This is usually the case for
multithreaded applications and distributed applications at large.

Experience indicates that logging was an important component of the
development cycle. It offers several advantages. It provides precise
context about a run of the application. Once inserted into
the code, the generation of logging output requires no human
intervention. Moreover, log output can be saved in persistent medium
to be studied at a later time. In addition to its use in the
development cycle, a sufficiently rich logging package can also be
viewed as an auditing tool.

As Brian W. Kernighan and Rob Pike put it in their truly excellent
book "The Practice of Programming":

As personal choice, we tend not to use debuggers beyond getting a
stack trace or the value of a variable or two. One reason is that it
is easy to get lost in details of complicated data structures and
control flow; we find stepping through a program less productive
than thinking harder and adding output statements and self-checking
code at critical places. Clicking over statements takes longer than
scanning the output of judiciously-placed displays. It takes less
time to decide where to put print statements than to single-step to
the critical section of code, even assuming we know where that
is. More important, debugging statements stay with the program;
debugging sessions are transient.

Logging does have its drawbacks. It can slow down an
application. If too verbose, it can cause scrolling blindness. To
alleviate these concerns, log4j is designed to be reliable, fast and
extensible. Since logging is rarely the main focus of an application,
the log4j API strives to be simple to understand and to use.

Log4j 2

Log4j 1.x has been widely adopted and used in many applications. However,
through the years development on it has slowed down. It has become more
difficult to maintain due to its need to be compliant with very old versions
of Java and became
End
of Life in August 2015.
Its alternative, SLF4J/Logback made many needed improvements to the
framework. So why bother with Log4j 2? Here are a few of the reasons.

Log4j 2 is designed to be usable as an audit logging framework. Both Log4j
1.x and Logback will lose events while reconfiguring. Log4j 2 will not. In
Logback, exceptions in Appenders are never visible to the application. In
Log4j 2 Appenders can be configured to allow the exception to percolate
to the application.

Log4j 2 contains next-generation Asynchronous Loggers based
on the LMAX Disruptor library.
In multi-threaded scenarios Asynchronous Loggers have 10 times higher throughput and
orders of magnitude lower latency than Log4j 1.x and Logback.

Log4j 2 is garbage free for
stand-alone applications, and low garbage for web applications during steady state logging.
This reduces pressure on the garbage collector and can give better response time performance.

Due to the Plugin system configuration is simpler. Entries in the configuration
do not require a class name to be specified.

Support for custom log levels.
Custom log levels can be defined in code or in configuration.

Support for lambda expressions.
Client code running on Java 8 can use lambda expressions to lazily construct a log message only if
the requested log level is enabled. Explicit level checks are not needed, resulting in cleaner code.

Support for Message objects. Messages allow support for interesting and
complex constructs to be passed through the logging system and be efficiently
manipulated. Users are free to create their own
Message
types and write custom Layouts, Filters and
Lookups to manipulate them.

Log4j 1.x supports Filters on Appenders. Logback added TurboFilters to allow
filtering of events before they are processed by a Logger. Log4j 2 supports
Filters that can be configured to process events before they are handled by
a Logger, as they are processed by a Logger or on an Appender.

Many Logback Appenders do not accept a Layout and will only send data in a
fixed format. Most Log4j 2 Appenders accept a Layout, allowing the data to
be transported in any format desired.

Layouts in Log4j 1.x and Logback return a String. This resulted in the problems
discussed at Logback Encoders.
Log4j 2 takes the simpler approach that Layouts always return a byte array. This has
the advantage that it means they can be used in virtually any Appender, not just
the ones that write to an OutputStream.

Log4j 2 takes advantage of Java 5 concurrency support and performs locking
at the lowest level possible. Log4j 1.x has known deadlock issues. Many of these
are fixed in Logback but many Logback classes still require synchronization at
a fairly high level.

It is an Apache Software Foundation project following the community and support
model used by all ASF projects. If you want to contribute or gain the right to
commit changes just follow the path outlined at
Contributing.